Efficient coupling of light into a light guides

ABSTRACT

Method and apparatus of coupling light into a light guide using error detecting and correcting systems. A rectangular light guide is mated with an X-axis optical fiber and with a Y-axis optical fiber. An X-axis photodetector senses light that couples from the light guide into the X-axis optical fiber, while a Y-axis photodetector senses light that couples from the light guide into the Y-axis optical fiber. An error detector compares the outputs of the X-axis and the Y-axis photodetectors to produces error signals that depend on the X-axis, the Y-axis, and the Z-axis positional errors. The errors signals are applied to an error correcting system that adjusts the relative position of the light guide and the light directed onto the light guide.

This invention relates to light guides that transport light. Morespecifically, this invention relates to image projection systems havinglight sources that produce light that is efficiently coupled into lightguides.

Some color image projection systems use a white light source whose beamis separated into primary components (usually red, blue and green),which are then individually modulated according to corresponding colorinformation derived from incoming display signals. Subsequently, themodulated color components are recombined to produce a full color imagethat is projected onto a viewing screen.

The modulation of the primary components is commonly performed using aseparate electro-optical light modulator, typically a liquid crystaldisplay (LCD) panel, for each primary component. Another type of colorimage projection system is similar, but uses only one LCD panel tomodulate the primary components. This is performed by shaping theprimary components into band-shaped cross-sections that are sequentiallyscrolled across an LCD panel that is modulated in accord with the colorcomponent being scrolled. Other types of light modulators, such as thosebased on micro-mechanical mirrors (MEMS), are also known.

The image projection systems described above require a light source,various optical elements such as prisms, polarizers, and lenses, anelectronic subsystem, and a modulator or modulators. While such imageprojection systems are generally successful, they tend to be relativelylarge. This is a problem because market demands are greater for lighter,more compact systems. Thus, it is desirable to reduce the size andweight of image projection systems. It should be noted that smallerimage projection systems tend to use smaller optical elements, which canbe significantly cheaper than larger optical elements.

While reducing the size of an image projection system is beneficial inmany respects, smaller image projection systems are susceptible tovarious problems. For example, reduced size image projection systemsbenefit from using light guides to internally transport light to thevarious optical components. While this itself is not a problem,efficiently coupling light into a light guide, particularly one having asmall cross-section, can be difficult to do. This is especially trueover time, temperature, and physical impacts to the image projectionsystem. Indeed, movement of a light source's emissive volume, such as achange in position of an arc relative to a light source's inputelectrodes, can significantly impact the efficiency of optical couplingbetween the light source and a light guide.

Similar problems of optically coupling a light source to a light guideare discussed in U.S. Pat. No. 5,319,195, which issued on Jun. 7, 1994to Jones et al. That patent teaches coupling a laser beam into anoptical fiber. FIGS. 1 and 2 of Jones, and the supporting text, teachtransducers that measuring the power of light propagating in an opticalfiber that transports a laser beam. Furthermore, FIGS. 3 and 5, and thesupport text, teach using those transducers to determine misalignmentbetween the laser and the optical fiber, and suggest using thedetermined misalignment to align the input end of the optical fiber withthe laser.

While beneficial, the teachings of Jones are inherently limited. Forexample, misalignment between the optical fiber and the laser is sensedusing only one input end transducer. Thus, directional informationregarding misalignment is either nonexistent or very limited. Thus,Jones does not suggest determining and correcting multi-directionalalignment errors or focal errors.

One difficulty in ensuring efficient optical coupling between a lightsource and a light guide is determining when the coupling is notefficient. Thus, an error detecting system that senses when light from alight source is not efficiently coupled into a light guide would beuseful. Beneficially, that error detecting system would providesufficient information to determine both the degree and the direction ordirections of the coupling errors. Then, the information from the errordetecting system could be used to adjust the relative position betweenthe light guide and the light directed into the light guide so as toachieve efficient coupling. Such as error correcting system would beparticularly useful in an image projection system to automaticallyensure efficient coupling of light from a light source, typically an arclamp, into a light guide. Such an image projection system would beeasier to initially align as well since only a coarse initial set-upwould be required since the system could be self-aligning.

Therefore, in view of the foregoing, it is desirable to provide for anerror detecting system that senses when light from a light source is notefficiently coupled into a light guide. Accordingly, an error detectingsystem is disclosed herewith that uses a rectangular (including square)light guide. A first (X-axis) optical fiber core is mated to one side ofthe light guide while a second (Y-axis) optical fiber is mated toanother (perpendicular) side. A first (X-axis) photodetector senseslight that couples from the light guide into the first (X-axis) opticalfiber while a second (Y-axis) photodetector senses light that couplesfrom the light guide into the second (Y-axis) optical fiber. It shouldbe noted that, within limits, the light that couples into the opticalfibers increases as the coupling efficiency between the light source andthe light guide decreases.

An electronic error detector then compares the outputs of the first andsecond photodetectors to determine whether the light beam from the lightsource is efficiently coupled into the light guide. Beneficially, theelectronic error detector produces X-axis, Y-axis, and Z-axis positionalerrors. Also beneficially, the error detecting system providessufficient information to determine both the degree and the direction ordirections of the coupling error.

The output of the error detecting system can then be used by an errorcorrecting system to adjust the relative position of the light guide andthe light that is directed into the light guide so as to achieveefficient coupling. The error correcting system can adjust the relativeposition by incorporating a number of different types of relative motioninducing devices, specifically including motors, piezoelectric benders,opto-electronic modulators, opto-electronic light valves, diffractiongrating, and electromechanical devices such as solenoids and voice-coilschemes.

The error detecting system and the error correcting system describedabove can also be used in image projection systems to automaticallyensure efficient coupling of light from a light source, typically an arclamp, into a light guide. Such an image projection system would beeasier to initially align as well since fine alignment can be performedautomatically.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only.Other embodiments, variations of embodiments, and equivalents, as wellas other aspects, objects, and advantages of the invention, will beapparent to those skilled in the art and can be obtained from a study ofthe drawings, the disclosure, and the appended claims, or may be learnedby practicing the invention.

In the drawings:

FIG. 1 illustrates a generic color image projector system that is inaccord with the principles of the present invention;

FIG. 2 illustrates an error detecting system suitable for use in theimage projector system of FIG. 1; and

FIG. 3 illustrates an error correcting system suitable for use in theimage projector system of FIG. 1.

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingfigures.

FIG. 1 illustrates an image projector system 10 that is in accord withthe principles of the present invention. It should be understood thatthe image projector 10 represents a generic image projector that issuitable for explaining the principles of the present invention and forillustrating how those principles can benefit image projection systemsin general.

The purpose of the image projector system 10 is to project a modulatedlight beam 12 onto a surface 14 so as to create desired images. Thoseimages are produced in accord with input signals, such as televisionsignals, computer generated signals, or other types of digitized oranalog signals, that are input on a port 16.

The image projector system 10 includes an illumination unit 18 thatincludes a light source 20 and a reflector/lens system 22. In practice,arc lamps are often used as light sources. The reflector/lens system 22inputs light into a rectangular (including square) light guide 24. Thelight in the light guide 24 is transported to a color separator 26 thatseparates the white light from the light source 20 into its red, green,and blue components (or other primary components). The red component isapplied via a light guide 28 to a red modulator 40. The green componentis applied via a light guide 34 to a green modulator 36, and the bluecomponent is applied via a light guide 38 to a blue modulator 30.

Still referring to FIG. 1, the signals input on port 16 are applied toan electronic subsystem 42. That subsystem extracts red, green, and blueinformation from the input signals to produce red, green, and bluemodulation signals. The red modulation signals are applied to the redmodulator 40 via a red signal line 46, the green modulation signals areapplied to the green modulator 36 via a green signal line 48, and theblue modulation signals are applied to the blue modulator 30 via a bluesignal line 50. The red, green, and blue modulators (40, 36, and 30)modulate the color components in accord with the modulation signals.

The modulated color components are then applied to an optical processor56 that combines the red, green, and blue modulated color components toproduce the light beam 12. The light beam 12 passes through a set ofprojection optics 57 to produce the desired images on the surface 14.

The image projector system 10 uses numerous light guides (the lightguides 24, 28, 34, and 38,) to internally transport light. While the useof light guides is beneficial, in practice it is important to ensurethat light is efficiently coupled into the various light guides. To doso, the image projector system 10 includes at least one error detectingcircuit and at least one error correction system.

Referring now to FIGS. 2 and 3, the error detecting circuit includes aY-axis optical fiber 102 and an X-axis optical fiber 104. Each opticalfiber has a core 106 that is surrounded by a cladding layer 108 (seeFIG. 3). Normally, the cladding layer 108 is surrounded by a bufferlayer and by a jacket that protect the core 106 and the cladding layer108 and that provide mechanical strength. The Y-axis and X-axis opticalfibers 102 and 104 mate to a rectangular light guide, referenced inFIGS. 2 and 3 as light guide 110. It should be understood that the lightguide 110 generically represents one or more of the light guides shownin FIG. 1.

As shown in FIG. 3, the light guide 110 includes a core 112 that issurrounded by an outer cladding layer 114. The outer cladding layers 108of the Y-axis and X-axis optical fibers 102 and 104 are partiallyremoved to enable their cores 106 to contact the outer cladding layer114. Beneficially, the outer cladding layer 114 and the cores 106 havesimilar refractive indexes. This enables light in the outer claddinglayer 114 to couple into the cores 106.

Referring now to both FIGS. 2 and 3, light 142 from a light source,assumed for explanatory convenience to be the light source 20, is to beefficiently coupled into the light guide 110. As shown, the light 142passes through the lens portion of the reflector/lens 22 to produce alight spot on the light guide 110. Ideally, that light spot illuminatesonly the core 112. However, as shown in FIG. 3, this is not always thecase. When the light 142 overlaps the cladding layer 114 some of thelight 142 passes into the cladding layer 114 and couples into the core106 of an optical fiber. FIG. 3 shows the light 142 illuminating thecladding 114 under the Y-axis optical fiber 102. Thus, some of the light142 couples into the Y-axis optical fiber 102.

Referring now to FIG. 2, the light that couples into the Y-axis opticalfiber 102 is detected by a Y-axis photodetector 150. Similarly, lightthat couples into the X-axis optical fiber 104 is detected by an X-axisphotodetector 152. The outputs of the photodetectors 150 and 152 areapplied to an electronic error detector 154. The electronic errordetector 154 process the information from the photodetectors 150 and 152to produce X-axis correction signals on a line 156, Y-axis correctionsignals on a line 158, and Z-axis correction signals on a line 159. Itshould be noted that the electronic error detector 154 can includeamplifiers, a microcontroller, memory circuits, comparators, A/D and D/Aconverters and the like.

The X-axis corrections signals are derived primarily from the outputs ofthe X-axis photodetector 152, while the Y-axis correction signals arederived primarily from the outputs of the Y-axis photodetector 150.However, the Z-axis correction signals are derived from the outputs ofboth the X-axis photodetector 152 and the Y-axis photodetector 150. Withreference primarily to FIG. 2, the Z-axis correction signals relate tothe focus of the light 142 on the light guide 110. If the light spot istoo large, denoting poor focus, then the outputs of the X-axisphotodetector 152 and of the Y-axis photodetector 150 are increased.Optimum focus is achieved when the sum of these signals are minimized.

Referring now to FIG. 3, the X-axis correction signals on the line 156are applied to an X-axis relative motion-inducing device 160, while theY-axis correction signals on the line 158 are applied to a Y-axisrelative motion-inducing device 162. If the electronic error detector154 determines that the light 142 is off-center in the X-direction,X-axis correction signals are produced that cause the X-axis relativemotion-inducing device 160 to move the light spot produced by the light142 toward the center of the core 112 along the X-axis. Likewise, if theelectronic error detector 154 determines that the light spot isoff-center in the Y-direction, Y-axis correction signals are producedthat cause the Y-axis relative motion-inducing device 162 to move thelight spot toward the center of the core 112 along the Y-axis. Referringnow to FIG. 2, in addition, the Z-axis correction signals on the line159 are applied to a Z-axis motion-inducing device 161. The Z-axismotion-inducing device 161 changes the focus of the light 142 on thelight guide 110. Thus, the image projection system 10 includes a closedloop, error-controlled feedback network that automatically adjusts theposition and focus of the light 142 such that the light 142 isefficiently coupled into the light guide 110.

It should be understood that the relative motion-inducing devices 160,161, and 162 do not have to physically contact the light guide 110 orthe light source 20. In fact, they can move a lens, a mirror, a prism orany other element that directs the light 142 into the light guide 110.FIG. 2 illustrates possible motions 170 of the reflector/lens 22 andpossible motions 172 of the light source 20. Furthermore, the relativemotion-inducing devices 160, 161, and 162 do not actually have to inducephysical motion. For example, the relative motion-inducing devices canbe opto-electronic positioning elements such as light valves ormodulators. Thus, suitable relative motion-inducing devices 160, 161,and 162 include such things as motors, solenoids, electromagneticelements, piezoelectric benders, light valves, diffraction gratings, andmodulators.

The embodiments and examples set forth herein are presented to explainthe present invention and its practical application and to therebyenable those skilled in the art to make and utilize the invention. Thoseskilled in the art, however, will recognize that the foregoingdescription and examples have been presented for the purpose ofillustration and example only. Other variations and modifications of thepresent invention will be apparent to those of skill in the art.Therefore, it is intended that the scope of the present invention bedefined by the claims appended hereto, giving full cognizance toequivalents in all respects.

1. An optical system, comprising: a rectangular light guide having aninner core and an outer cladding layer; a first optical fiber that isoptically coupled to a first side of the outer cladding layer; a secondoptical fiber that is optically coupled to a second side of the outercladding layer; a first photodetector for sensing light coupled into thefirst optical fiber; and a second photodetector for sensing lightcoupled into the second optical fiber.
 2. An optical system according toclaim 1, further including an error detecting system coupled to thefirst photodetector and to the second photodetector, the error-detectingsystem for producing both X-axis error correction signals and Y-axiserror correction signals based on light sensed by the firstphotodetector and by the second photodetector.
 3. An optical systemaccording to claim 1, further including an error detecting systemcoupled to the first photodetector and to the second photodetector, theerror-detecting system for producing Z-axis error correction signalsbased on light sensed by the first photodetector and by the secondphotodetector.
 4. An optical system according to claim 2, furtherincluding an X-axis relative motion-inducing device for moving a lightbeam relative to the inner core in response to the X-axis errorcorrection signals.
 5. An optical system according to claim 4, furtherincluding a Y-axis relative motion-inducing device for moving a lightbeam relative to the inner core in response to the Y-axis errorcorrection signals.
 6. An optical system according to claim 3, furtherincluding a Z-axis relative motion-inducing device for moving a lightbeam relative to the inner core in response to the Z-axis errorcorrection signals.
 7. An optical system according to claim 1, furtherincluding a light source for producing a light beam that is directedinto the light guide.
 8. An optical system according to claim 1, whereinthe first optical fiber includes a first core and a first claddinglayer, wherein the first cladding layer is partially removed such thatthe first core optically contacts the outer cladding layer.
 9. Anoptical system according to claim 8, wherein the first core and theouter cladding layer have such similar refractive indexes that light inthe outer cladding layer couples into the first core.
 10. An imageprojection system, comprising: a rectangular light guide having an innercore and an outer cladding layer; a light source for projecting a lightbeam into the light guide; a first optical fiber optically coupled to afirst side of the outer cladding layer; a second optical fiber opticallycoupled to a second side of the outer cladding layer; a firstphotodetector for sensing light coupled into the first optical fiber;and a second photodetector for sensing light coupled into the secondoptical fiber.
 11. An image projection system according to claim 10,further including an error detecting system that is coupled to the firstphotodetector and to the second photodetector, the error detectingsystem for producing X-axis error correction signals, Y-axis errorcorrection signals, and Z-axis error correction signals from the lightsensed by the first photodetector and by the second photodetector. 12.An image projection system according to claim 11, further including anX-axis relative motion-inducing device for moving the light beamrelative to the inner core in response to the X-axis error correctionsignals, a Y-axis relative motion-inducing device for moving the lightbeam relative to the inner core in response to the Y-axis errorcorrection signals, and a Z-axis relative motion-inducing device forfocusing the light beam in response to the Z-axis error correctionsignals.
 13. An image projection system according to claim 12, furtherincluding a color separator that separates light from the light sourceinto primary components.
 14. An image projection system according toclaim 13, further including a modulator system that modulates theprimary components in accord with modulation signals.
 15. An imageprojection system according to claim 14, further including an opticalprocessor that combines the modulated primary components to produceimage light beams.
 16. An image projection system according to claim 10,wherein the first optical fiber includes a first core and a firstcladding layer, wherein the first cladding layer is partially removedsuch that the first core optically contacts the outer cladding layer ofthe rectangular light guide, and wherein the first core and the outercladding layer have such similar refractive indexes that light in theouter cladding layer couples into the first core.
 17. A method ofcoupling light into a light guide, comprising: directing light into alight guide having a cladding layer; removing a first portion of thelight that is directed into the cladding layer, wherein the firstportion is removed along an X-axis; removing a second portion of thelight that is directed into the cladding layer, wherein the secondportion is removed along a Y-axis; sensing the first portion of thelight to determine an X-axis misalignment of the light; and sensing thesecond portion of the light to determine a Y-axis misalignment of thelight.
 18. A method of coupling light into a light guide according toclaim 17, further including using the first portion and the secondportion to determine a Z-axis misalignment of the light.
 19. A method ofcoupling light into a light guide according to claim 18, furtherincluding adjusting the relative position of the light and the lightguide such that the Y-axis, the X-axis, and the Z-axis misalignments arereduced.
 20. A method of coupling light into a light guide according toclaim 19, wherein adjusting the relative position of the light and thelight guide includes moving the light guide.
 21. A method of couplinglight into a light guide according to claim 20, wherein the light guideis moved in both the X-direction and the Y-direction.
 22. A method ofcoupling light into a light guide according to claim 20, whereinadjusting the relative position of the light and the light guideincludes moving the image of the light source.
 23. A method of couplinglight into a light guide according to claim 17, wherein the sensed firstand second portions of the light produce error-correcting signals thatcause the X-axis, the Y-axis, and the Z-axis misalignments to bereduced.